I am going to confidently assume that the laptop only has one 2.5" drive bay. If you want to put your z400s in, you will need to take out the 1TB sata ssd. The only way to keep your 1TB HDD is to get a different SSD all together, one that is an M.2.
https://rog.asus.com/wp-content/uploads/2016/03/SSDs-980x274.jpg - for reference, to get a visual of the different form factors.
and NO, there does not exist an to go from 2.5" SATA to m.2 or mSATA. adapters exist to go the opposite way, assuming the sdd in question is not a PCIe ssd.
Thanks, No it is a SATA SSD. Works fine in older laptops so the G3 doesn't have 2 SATA connections and I can't get an adapter so no upgrade for now. Alas, I have no cash for a new 256Gb m.2 drive.